The English works of Thomas Hobbes of Malmesbury, Volume 01 (of 11) — Thomas Hobbes — John Shaqi
The English works of Thomas Hobbes of Malmesbury, Volume 01 (of 11)
Thomas Hobbes · en
Note, that when the sector is a hemisphere, the axis of the cone
vanisheth into that point which is the centre of the sphere; and
therefore it addeth nothing to half the axis of the portion. Wherefore,
if in the axis of the hemisphere there be taken from the centre ¾ of
half the axis, that is, ⅜ of the semidiameter of the sphere, there will
be the centre of equiponderation of the hemisphere.
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[Illustration:
_Vol. 1. Lat. & Eng._
C.XXIII.
_Fig. 1-9_
]
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CHAPTER XXIV.
OF REFRACTION AND REFLECTION.
1. Definitions.—2. In perpendicular motion there is no refraction.—3.
Things thrown out of a thinner into a thicker medium are so
refracted that the angle refracted is greater than the angle of
inclination.—4. Endeavour, which from one point tendeth every way,
will be so refracted, as that the sine of the angle refracted will
be to the sine of the angle of inclination, as the density of the
first medium is to the density of the second medium, reciprocally
taken.—5. The sine of the refracted angle in one inclination is to
the sine of the refracted angle in another inclination, as the sine
of the angle of that inclination is to the sine of the angle of this
inclination.—6. If two lines of incidence, having equal inclination,
be the one in a thinner, the other in a thicker medium, the sine of
the angle of inclination will be a mean proportional between the two
sines of the refracted angles.—7. If the angle of inclination be
semirect, and the line of inclination be in the thicker medium, and
the proportion of their densities be the same with that of the
diagonal to the side of a square, and the separating superficies be
plane, the refracted line will be in the separating superficies.—8.
If a body be carried in a strait line upon another body, and do not
penetrate the same, but be reflected from it, the angle of
reflection will be equal to the angle of incidence.—9. The same
happens in the generation of motion in the line of incidence.
DEFINITIONS.
[Sidenote: Definitions.]
I. Refraction is the _breaking_ of that strait line, in which a body is
moved or its action would proceed in one and the same medium, into two
strait lines, by reason of the different natures of the two mediums.
II. The former of these is called the _line of incidence_; the latter
the _refracted line_. III. The _point of refraction_ is the common point
of the line of incidence, and of the refracted line.
IV. The _refracting superficies_, which also is the _separating
superficies_ of the two mediums, is that in which is the point of
refraction.